It was common practice to establish a conference connection of n telephone lines using space division technology. This technology uses metallic contacts, contained in an array of relays or cross-bar type switches, to establish a telephone connection between a first subscriber and a second subscriber. Often, as additional subscribers were added to the space division connection, the voice quality of the connection was degraded due to the loading of the individual lines on the connection.
This problem was alleviated somewhat by limiting the number of connected conferees and by using operational amplifiers with resistor networks to build low impedance summing bridges.
Currently, it is common for telephone connections to pass through some form of digital time shared circuitry or channel, such as the Bell System's well known "T1" carrier system, a time division multiplexing switching system.
Time division multiplexing is a type of digital switching in which a channel is time shared among multiple users, and in which multiplexed time slots are allocated for establishing communication sessions.
The art of digital time division switching also includes systems known as Time Slot Interchangers (TSI), in which digital message samples from a first input time slot are switched to a second output time slot. TSI is now used to establish two party connections.
U.S. Pat. No. 4,119,807; entitled "Digital Time Division Multiplex Switching System" and issued Oct. 10, 1978; is an example of a time shared digital switch arranged to establish conference connections without restriction as to the number of conferees that may participate in a conference session. U.S. Pat. No. 4,119,807 discloses an arrangement for sequentially summing digital message samples from 128 lines connected respective to 128 time slots during a first time frame and outputting the summed samples to each line of the connection, respective to its assigned time slot, during a second time frame.
A digital switching arrangement of this type uses two summation memories, which are alternately loaded and unloaded respective to a time frame. In addition, third and fourth memories are alternately loaded in step with the summation memories, and are used for the storage of digital message samples respective to assigned time slots. Each summation memory is cleared at the beginning of its respective storage cycle or time frame in preparation for the storage of a new series of summed message samples.
Since each time frame is a storage cycle, a pause is created at the beginning of each time frame in order to clear one of the two summation memories. Hence, there is a corresponding pause in supplying message samples to an output terminal. Such a pause in the output stream is not compatible with full utilization of TDM channel capacity since most digital systems are not designed to tolerate a pause in the data stream. Further, this memory reset technique is not compatible with high density high speed memory operations for the rest of the switch requiring a wate to every summation memory during the pause.